1. Field of the Invention
The present invention relates generally to fiber optics. In particular, the present invention relates to a precision optical filter with a ball-end joint.
2. The Prior Art
Background
Fiber optic communication systems are largely responsible for the recent expansion of bandwidth in communications systems such as the Internet. Much of this bandwidth expansion is being accomplished through the multiplexing of many. channel of optical signals onto a single optical fiber. In typical applications today, as many as 128 semiconductor lasers may share a single fiber through a process known as xe2x80x9cwavelength division multiplexingxe2x80x9d (WDM). A Wavelength Division Multiplexer (WDM) used to accomplish this process is typically a passive optical device configured to combine multiple channels of optical information onto a single optical fiber (multiplexing), or to separate multiple channels of optical information contained in a single optical fiber onto separate optical fibers (demultiplexing).
Much attention today is being given to Dense Wavelength Division Multiplexers (DWDM) because of the requirement to divide signals that are spaced very close together in wavelength. Current International Telecommunications Union (ITU) specifications call for channel separations of approximately 0.4 nm. Using such channel separations, a single fiber may carry as many as 128 channels.
Because of this close channel separation, it is desirable to fabricate DWDMs with excellent stability over both time and temperature. It is desirable, among other things, that the insertion loss of a DWDM may not vary more than +/xe2x88x920.5 dB over the temperature range of 0xc2x0-70xc2x0 C. Additionally, since a DWDM usually comprises many filters cascaded together. For example, an 8-channel DWDM may comprise 8 cascaded filters. Thus, at the final stage of such an 8-channel DWDM, any variation introduced by a particular channel may be multiplied by as much as eight times. Accordingly, it is desirable to keep any such variations to a minimum. However, it is a challenge for typical DWDMs produced today to meet such standards.
FIG. 1 shows a DWDM 100 of the prior art. DWDM 100 includes a first collimator 102which further includes a ferrule 104 and a lens 106. Ferrule 104 further includes an incident fiber 110 and a reflecting fiber 112. Typically, lens 106 comprises a GRIN lens standard in the art. DWDM 100 further includes a thin-film filter 114 standard in the art. Additionally, DWDM 100 includes a second collimator 116, which also includes a lens 118 and a ferrule 120 having a transmitting fiber 122. During fabrication, first collimator 102, thin film filter 114, and second collimator 116 must be precisely aligned about an axis 106 to function properly. A finished DWDM as shown in FIG. 1 is typically 5 mm in diameter and 40 mm in length.
FIGS. 2A, 2B, and 2C are diagrams which provide a brief overview of the operation of a DWDM. FIGS. 2A-2C are graphical diagrams having a transmission axis T versus wavelength xcex. FIG. 2A shows a plurality of channels incident to a DWDM in which it is desired to isolate channel xcex0. By way of example, the signals of FIG. 2A may be applied to incident fiber 110 of DWDM 100 of FIG. 1. If the DWDM 100 of FIG. 1 is aligned properly, the incident signals will be optically coupled through lens 106 to the filter 114. Filter 114 is coated with a multi-layer coating standard in the art to reflect all of the incident signal into the reflecting fiber 112, except for the channel xcex0, as is shown in FIG. 2B. Furthermore, filter 114 will transmit the channel xcex0 through lens 118 and ultimately to transmitting fiber 122, as shown in FIG. 2C.
However, as mentioned above, the DWDM must be properly aligned for the results of FIGS. 2A-2C to occur. The position of the thin-film filter is critical to the proper operation of the DWDM, with acceptable errors in orientation being in the order of 1 milliradian or 1 micrometers. Two challenges face manufacturers of DWDMs: first, the DWDM must be precisely aligned and secured during manufacturing; and, secondly, the DWDM must be able to tolerate the temperature ranges imposed by environmental and operational conditions and tested through the Bellcore process.
FIG. 3 is a detail diagram of a prior art diagram of a prior art DWDM. FIG. 3 focuses on how the thin film filter 114 of DWDM 100 shown in FIG. 1 is secured in prior art devices. During fabrication, the filter 114 is aligned such that the incident signals are properly reflected and transmitted as described above. When the desired location of the filter 114 is determined, it is typically secured with epoxy (shown as spots of epoxy 300) to the collimator 102. It should be noted that FIG. 3 is not drawn to scale and the tilt of the filter is exaggerated for illustrative purposes.
However, the process of securing the filter with epoxy as shown in FIG. 3 has been shown to introduce certain deficiencies in the operation of the DWDM. Often, when the filter is secured in place, there is a significant gap between the filter and the collimator, and when the epoxy is applied to the filter, the epoxy spreads and dries in an uneven manner. This uneven distribution of epoxy can lead to deficiencies in the final product. For example, epoxy has been shown to expand and contract with temperature changes, thus causing the DWDM to shift operationally with temperature. Naturally, any temperature deviations will reflect negatively in the Bellcore tests and performance in the field. These deviations can be exaggerated if the epoxy is distributed nonuniformly. Additionally, it has been shown that when epoxy makes contact with the delicate surface of a thin-film filter, the areas proximate to the epoxy may suffer performance degradations.
Hence, there is a need for a method and apparatus for aligning and securing a thin-film filter within an optical device which overcomes the problems of the prior art.
The invention satisfies the above needs. The present invention relates generally to fiber optics. In particular, the present invention relates to a precision optical filter with a ball-end joint.
A portion of an optical device is disclosed. In one aspect of the present invention, the device comprises a cylinder formed about an axis having first and second ends, the second being formed so as to define a segment of an inward-facing concave spherical surface;
a module defining a cylinder formed about the axis and having first and second ends, the module having an optical element disposed therein about the axis, the first end of the module being formed so as to define a segment of an outward-facing convex spherical surface, the convex surface being complimentary in shape to the concave surface; and
wherein the complimentary concave and convex surfaces of the cylinder and the module being mated so as to allow the optical element to be aligned about a plane forming a predetermined angle with the axis.
Additional aspects of the present invention include the use of the present invention in optical devices such as DWDMs.
A method for aligning an optical element is disclosed. In a preferred embodiment, the method comprises: providing a cylinder formed about an axis having first and second ends, the second end being formed so as to define a segment of an inward-facing concave spherical surface;
providing a module defining a cylinder formed about the axis and having first and second ends, the module having an optical element disposed therein between the first and second ends of the module about the axis, the first end of the module being formed so as to define a segment of an outward-facing convex spherical surface, the convex surface being complimentary in shape to the concave surface;
mating the complimentary concave and convex surfaces of the cylinder and the module; and
wherein the optical element may be aligned about a plane forming a predetermined angle with the axis.
An optical device is disclosed. In a preferred embodiment, the device comprises: a first collimator, the first collimator formed about an axis and defining a cylinder having first and second ends, the first collimator further having a ferrule disposed in the first end of the collimator about the axis, a first lens disposed in the first collimator between the first and second ends about the axis, and the second end of the first collimator being formed so as to define a segment of an inward-facing concave spherical surface;
a module defining a cylinder formed about the axis and having first and second ends, the module having an optical element disposed therein between the first and second ends of the module about the axis, the first end of the filter module being formed so as to define a segment of an outward-facing convex spherical surface, the convex surface being complimentary in shape to the concave surface;
a second collimator, the second collimator formed about the axis and defining a cylinder having first and second ends, the second collimator further having a ferrule disposed in the second end of the collimator about the axis, a second lens disposed in the second collimator between the first and second ends about the axis, the second collimator being optically coupled to the module; and
wherein the complimentary concave and convex surfaces of the first collimator and the module being mated so as to allow the optical element to be aligned about a plane forming a predetermined angle with the axis.
Additional aspects of the present invention include utilizing an optical element such as an optical filter such as a band pass filter, a long pass filter, a short pass filters, a selective filter, a GRIN lens, a spherical lens, or an aspherical lens. The present invention may also include a thin-film filter.
The present invention may also be used as a portion of a DWDM.